Electronic engine controllers (EEC) typically utilize a closed-loop form of control to maintain the ratio of air and fuel in an air/fuel mixture combusted by the engine at or near stoichiometry. Under closed-loop control, the amount of fuel delivered is determined primarily by the concentration of oxygen in the exhaust gas. The oxygen concentration being indicative of the ratio of air and fuel that has been ignited. Typically, an oxygen sensor detects the concentration of oxygen in the exhaust gas and transmits a representative signal to the EEC which determines a fuel flow rate in response to the detected oxygen concentration.
Modern automotive engines utilize a three-way catalytic converter to reduce the unwanted by-products of combustion. Typically, closed-loop air/fuel control systems control the fuel flow rate to cause an oscillation of the air/fuel mixture about stoichiometry in an attempt to maximize catalytic converter efficiency. A number of variables affect converter efficiency including the average ratio of air to fuel combusted by the engine, the limit cycle frequency of the fuel delivery rate and the peak to peak amplitude of the fuel delivery rate.
Sometimes, in order to achieve peak converter efficiency, it is desirable to bias the ratio of air and fuel to be either rich or lean of stoichiometry. For instance, the use of fuels other than gasoline, such as methanol, natural gas or liquid propane, can cause an oxygen sensor to transmit a signal which indicates an oxygen content which is offset from the actual oxygen content of the detected oxygen gas. In such an instance it is desirable to calibrate the engine to compensate for the offset.
Known engine controllers are limited in the manner in which they can be calibrated in order to achieve maximum catalytic converter efficiency. Consequently, either the engine hardware, such as an oxygen sensor in the case of the use of alternative fuels, must be modified to meet the specifications of the engine controller, or the engine controller must be redesigned to meet the specifications of the different fuel type or different hardware. This limitation causes increased vehicle design costs and limits the component complexity.